Hydrodynamically modulated hull cell

ABSTRACT

Electrodeposition in surface finishing as well as in electroforming can be performed in a broad range of hydrodynamic conditions. In some instances, such as barrel plating, the liquid is barely moving relative to the work piece, while in jet plating the solution moves up to several meters per second relative to the piece to be plated. In such a wide range of hydrodynamic conditions, the selection of appropriate quality control (QC) methods for each particular operation is very critical. The QC method has to resemble the manufacturing operation in terms of the hydrodynamic conditions otherwise the quality control and the functional test for the bath performance have no meaning. Available Hull Cell systems have a limited capability in varying the liquid velocity. At best a relative velocity of up to 20 or perhaps 30 cm/sec can be achieved with uneven distribution. 
     The invention provides, in electroplating QC methods for manufacturing, better similarity and relevance between a Hull Cell QC test and a particular plating operation. The invention is a simple and yet functional instrument which can be used to identify the performance of the plating bath prior to manufacturing operation under similar hydrodynamic conditions. The instrument is a rotating cylinder with a flexible Cu test panel attached to its surface. Like a Hull Cell, a range of current density can be simultaneously applied. Unlike a Hull Cell, the rotating speed of the cylinder and hence the solution agitation is practically unlimited. The operator can identify the operating window for the particular process and apply them to the production line. The instrument has also demonstrated usefulness in developing proprietary electroplating chemistries.

This is a continuation of application Ser. No. 07/550,266, filed Jul. 9,1990, now U.S. Pat. No. 5,228,976.

FIELD OF THE INVENTION

This invention concerns with an apparatus for control ofelectrodeposition performance of a plating bath prior to and duringmanufacturing operation.

DESCRIPTION OF THE PRIOR ART

Electroplating encompasses many scientific disciplines includingelectrochemistry, hydrodynamics and transport phenomena, organic andinorganic chemistry, materials science, and metallurgy, which, in turn,involves an appropriate selection of testing equipment and operatingparameters. One of the most critical parameters of electroplating is theinterfacial transport and its relation to current density. Once thechoice for the bath composition has been narrowed down, the definitionand the selection of current densities and transport conditions becomecritical factors which determine the bath performance and the platedproduct properties. To this end, several advanced techniques are beingused in electrochemical research and development.

Rotating electrodes, such as rotating discs, disc-rings, ring-ringelectrodes and rotating cylinders, create a defined hydrodynamiccondition in the operating cell while the electrochemical measurementsare being performed giving, in each experiment, information on a singlecurrent density. However, with few rare exceptions, the range of currentdensities in plating processes is typically wide. Ten to twenty percentvariation from the nominal current density for more simple and up to 200percent for more complex parts command a wide range of functionalefficiency of the plating process. Under such conditions, informationneeded concerning the effect of a broad range of current densitiesrequires numerous tests.

Hull cell, developed by R. O. Hull in 1939 as a quality control anddevelopmental tool, permits in a single test to preview a range ofcurrent densities which provide a desired plating characteristic at agiven total current. The Hull cell is described in R. O. Hull,"Proceedings of American Electroplaters Society", 27 (1939), pp 52-60and in U.S. Pat. No. 2,149,344 issued to R. O. Hull on Mar. 7, 1939.Also see a book by Walter Nohse et al. entitled "The Investigation ofElectroplating and Related Solutions with the Aid of the Hull Cell",Robert Draper Ltd., Teddington, England, 1966, especially pages 17-25.

The Hull cell is schematically represented in FIG. 31. The Hull cell,generally designated as 310, is a four-sided, fiat bottomed container,311, of certain capacity (typically in mls.) in which fiat verticallypositioned an anode, 312, and a cathode, 313, arc arranged at an angleeach to another. Typically, cathode is also at an angle to both of twoside walls, 314 and 315, of the container. The test panel is onlypartially submersible in an electrolyte, 316. Several sizes of Hullcells are commercially available with solution capacities of 250, 267,320, 534 and 1000 mls. The unique feature of the Hull cell, due to thearrangement of the anode to cathode geometry, is its ability toelectrodeposit on the test panel a metal across a range of currentdensities depending on the total applied current. The current densitiesrange from low current density at that end of cathode 313 which forms anacute angle with wall 314 to high current density at the end of thecathode forming an obtuse angle with wall 315. In FIG. 2 is representeda theoretical current density distribution in a typical standard Hullcell with applied total current of 2A.

The characteristic design feature of the Hull cell is the acute anglebetween anode 312 and cathode 313 combined with the effect of cell wall314 making the acute angle to the cathode at the low current density endportion of cathode 313. The angle between the electrodes and theshielding of the cell wall 314 provide the current devity distribution.At the low current end where wall 314 is at an acute angle to thecathode, the primary current density is infinitely small. Towards thecenter of the panel the current increases gradually reaching its maximumat the high current density end. A pattern of deposits obtained at thiswide range of current densities is used as an indicator of the qualityof the product that can be obtained if the plating is performed at anynominal current density within the given range. In a single test one canselect a range of current densities which provide desired platingcharacteristics. However, the use of the Hull cell is limited,especially under high liquid velocity conditions, due to the lack ofmass transfer control, uneven deposition rate at specified currentdensities and lack of high speed liquid mixing. In an experiment withvarying hydrodynamics and mass transport along the panel, the obtainedpatterns could be a result of both current density and hydrodynamicconditions. Such a result would be irrelevant to the operatingconditions and could not be used as a guide in bath performanceevaluation.

A typical range of current densities and mixing conditions for aspecific electroplating process is usually defined in the operatingmanual for that process. In order to keep the production quality at thespecified level, operators perform a quality control tests at regularintervals employing the Hull cell. Based on such tests, correctiveactions are applied to maintain the bath electrodeposition performance.Unfortunately, regardless of many common denominators, in most instancesthe pattern obtained is a result of vaguely defined variables such asthe mixing speed, location of the stirring bar, preparation of thesample and others.

Irreproducible and limited solution agitation is usually provided in aHull cell by a fixed rate reciprocating paddle, a magnetic stirrer or aforced gas mixing near the cathode/solution interface. By using such anarrangement, it is difficult to obtain consistently reproducible resultsand to correlate these to a manufacturing operation where the solutionagitation can vary significantly depending upon the application and thedesired plating rate. For example, in barrel plating operations thesolution is barely moving relative to the work piece while in jetplating applications the solution velocity can reach several meters persecond. Over such a wide range of hydrodynamic conditions, the effectsof chemical equilibria, additives, contaminants and other components ofthe plating solution on the quality of the product can varyconsiderably.

Particularly important is the interpretation of the effects ofimpurities on the pattern appearance such as peeling, dullness, or blackdeposit at low current density end of the panel. These symptoms willvary depending on the mixing conditions of the cell. However, the Hullcell apparatus is limited in providing information regarding a specificplating process which may be affected by variations in solutionagitation.

Furthermore, in many instances, particularly in modern platingfacilities, high speed plating baths with liquid velocity of severalmeters per second are being employed. For such conditions, informationdeveloped on the basis of the interpretation of a Hull cell panelobtained at typical Hull cell liquid velocities of 20-30 cm/sec. couldbe meaningless. Use of the Hull cell with higher mixing rates leads tospillage and usually gives little clue on the solution performance.

Another uncertainty is connected with inconsistencies in current densitydistribution. Hull cell panels are usually interpreted with the scale ofcurrent densities that are attributed to the position on the panel. Theposition of the liquid meniscus on one side and the vicinity of the cellbottom on the other side of the panel may influence the rate ofdeposition. As a result, the thickness distribution at each currentdensity designation can vary considerably leading to erroneousinterpretation. Such variations result from an uneven currentdistribution which is inherent to the Hull cell design and cansignificantly affect the reproducibility and the accuracy of panelinterpretation leading to losses in production and disorientation in R &D efforts.

Unfortunately, there are no viable alternatives to the Hull cell, today.Therefore, it is desirable to have a device which combines, in a singleunit, controlled cell hydrodynamics and resulting uniformity of masstransport at the panel interface with the capability of measuring widerange of current densities on a single experiment. Only in suchexperimental conditions, patterns obtained on the panel would reflectthe expected performance of the process at specified mixing rate.

SUMMARY OF THE INVENTION

This invention is a Hydrodynamically Modulated Hull cell (hereinafterreferred to as "HMH cell") which can be used for quality control and fordevelopmental applications and which combines, in a single unit, thecapability of providing for well-defined cell hydrodynamics andmeasuring wide range of current densities on a single experiment.

The HMH cell utilizes an elongated, cylindrically shaped measuringinstrument in which a cathode and an anode are vertically spaced along acentral longitudinal axis of the instrument, forming a single coaxiallyarranged unit with the anode being below the cathode. The instrument iscapable of being rotated about its longitudinal axis. This permits oneto vary in a controlled manner the solution agitation while effectivelyapplying a panel size and current density range typical for a standardHull cell. This apparatus can be used to establish the functionality ofelectroplating solutions in the liquid velocity range of the electrolytefrom quasi stationary to up to several meters per second.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a HMH cell;

FIG. 2 is a schematic representation of a HMH cell with a partiallysubmerged test panel;

FIG. 3 is a schematic representation of an electric field distributionin the HMH cell of FIG. 2;

FIG. 4 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 2;

FIG. 5 is a schematic representation of a HMH cell with a partiallysubmerged test panel and one horizontal baffle;

FIG. 6 is a schematic representation of an electric field distributionin the HMH cell of FIG. 5;

FIG. 7 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 5;

FIG. 8 is a schematic representation of a HMH cell with a partiallysubmerged test panel and two horizontal baffles;

FIG. 9 is a schematic representation of an electric field distributionin the HMH cell of FIG. 8;

FIG. 10 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 8;

FIG. 11 is a schematic representation of a HMH cell with a partiallysubmerged test panel and three horizontal baffles;

FIG. 12 is a schematic representation of an electric field distributionin the HMH cell of FIG. 11;

FIG. 13 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 11;

FIG. 14 is a schematic representation of a HMH cell with an insulatingcone shield;

FIG. 15 is a schematic representation of an electric field distributionin the HMH cell of FIG. 14;

FIG. 16 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 14;

FIG. 17 is a schematic representation of a HMH cell with a completelysubmerged test panel;

FIG. 18 is a schematic representation of an electric field distributionin the HMH cell of FIG. 17;

FIG. 19 is a schematic representation of a current density distributionalong a test panel in HMH cell of FIG. 17;

FIG. 20 is a schematic perspective representation of a preferredembodiment of the HMH cell with three horizontal baffles;

FIG. 21 is a schematic representation of a preferred embodiment of themeasuring instrument being used in the HMH cell;

FIGS. 22, 23 and 24 are a sequence of three charts presenting depositthickness distribution on a test panel deposited in a HMH cell withthree horizontal baffles at positions corresponding to 4, 20 and 80 ASF,respectively, and a total applied current of 2A;

FIGS. 25, 26 and 27 are a comparison of burned deposit on a high currentdensity portion of a test panel in a standard Hull cell (FIG. 25) withthat on a test panel in a HMH cell at 10 and 100 cm/s liquid velocity(FIGS. 26 and 27, respectively) from an aqueous solution containing 25g/l Pd at 40° and with a pH 7.5;

FIGS. 28, 29 and 30 are a schematic comparison of impurities effect on alow current density portion of a test panel in a standard Hull cell(FIG. 28) with that of a test panel in a HMH cell at 10 and 100 cm/sliquid velocity (FIGS. 29 and 30, respectively) from an aqueous solutioncontaining nickel sulfamate at 45° C., with pH=4.0 and with 100 ppm ofcopper as an impurity;

FIG. 31 is a schematic representation of the standard Hull cell;

FIG. 32 is a schematic representation of a current density distributionalong a test panel in a standard Hull cell;

FIGS. 33, 34 and 35 are a sequence of three charts presenting depositthickness distribution on a test panel deposited in a standard Hull cellat positions corresponding to 4, 20 and 80 ASF, respectively, and atotal applied current of 2A.

DETAILED DESCRIPTION

The HMH cell is schematically presented in FIG. 1. The cell, 10,includes a container, 11, for holding electrolyte, 12, and an elongated,cylindrically-shaped measuring instrument, 13, suspended vertically inthe container. The instrument is capable of being rotated about itslongitudinal central axis.

Instrument 13 includes an elongated cylinder, 15, of a suitableinsulating material, a sleeve-like metal cathode, 16, arranged coaxiallyon the cylinder, and a metal anode, 17, positioned at a lower end ofcylinder 15 coaxially with it and in a longitudinally spaced relation tothe cathode. Two sliding electrode contacts C slip rings"), 18 and 19,on the cylinder are provided above cathode 16 as current collectors forthe cathode and the anode, respectively. Upper part of cylinder 15 issecurable to an arm, 21, of a support arrangement (not shown). Ann 21supports instrument 13 suspended within container 11 and includes acover 22 and a suitable drive means, such as a belt drive or a motor(not shown). Cover 22 encloses container 11 and assists in positioninginstrument 13 coaxially of container 11. Drive means is for providingrotation to instrument 13 about its longitudinal central axis at adesired rate. The rate of rotation may be selected to be within a rangefrom 10 to 10,000 RPM.

Cathode 16 includes a flexible test panel, 212, (FIG. 21) removablyaffixable to cylinder 15. This may be accomplished in various ways, twoof which are as follows. In one instance opposite vertical ends of aflexible test panel may be inserted into a vertical longitudinal slot(not shown) in the surface of cylinder 15 while conforming the panel tothe circumference of the cylinder. Alternatively, horizontal ends of thetest panel conformed to the circumference of the cylinder may be securedby means of suitable fasteners (not shown). Such fasteners may includerubber bands which are slid over end portions of the panel. Electricalcontact to the test panel may be provided by at least one conductivecontact embedded into the surface of the cylinder, such as an area, ating or a sleeve. The contact area, ting or sleeve is electricallyconnected to slip ring 18, such as via a conductor, 213 (FIG. 21),placed internally of the cylinder.

Anode 17 is positioned below cathode 16 and is spaced from it by meansof a relatively short length spaces section 14, of insulating cylinder15. The spacing should not be less than that required to avoid unduebubbling at the space between the anode and the cathode. An excessivespacing is undesirable since it could adversely affect electric fielddistribution. A spacing of from 2 to 20 mm is useful, with 5-10 mm beingpreferred. Anode 17 is shown in FIG. 1 as being secured near the end ofcylinder 15. Anode 17 may be secured to the cylinder in any suitablemanner. This may involve a metallic fastener, such as a bolt exposed tothe electrolyte or hidden within a removable insulating disc or aninsulating disc provided with a threaded portion for mating with itscounterpart in cylinder 15. The type of fastening anode 17 to thecylinder is not important so long as anode 17 is supplied with energy,for example, via slip ring 19. This may be accomplished by connectinganode 17 and slip ring 19 by a conductor (not shown) placed internallyof cylinder 15. In FIG. 1, anode 17 is shown in the form of a thin disc,the radius of which is from 2 to 5 mm greater than the radius ofcylinder 15. Other sizes and forms of anode 17 may be useful. Forexample, radial dimension of the disc may be equal or even smaller thanthat of cylinder 15. Also, the thickness of anode 17 may vary over arelatively large range. The important prerequisite is the symmetricgeometry of anode 17 about the longitudinal axis of cylinder 15 in orderto distribute the electric field uniformly about the anode.

Cylinder 15 may be provided with an internal rod or shaft placed alongthe longitudinal axis of the cylinder (e.g., see FIG. 21). The rod orshaft may be needed to reinforce the cylinder and to provide means forimparting rotation to the cylinder. A metal rod or shaft may act as aconductor for connecting anode 17 to slip ring 19.

Cylinder 15 is made of an insulating material which is not adverselyaffected by the plating reaction, is non-conducting andnon-contaminating with respect to the solution and the resultant metaldeposit. Suitable material for the cylinder may be selected from suchinsulating materials as epoxy, polyethylene, polypropylene, polyvinylchloride, teflon, fiberglass and other plastic materials possessing theabove qualities. Container 11 and cover 22 are also of a material whichis resistant to the effects of the electroplating solution and isnon-conductive and non-contaminating with respect to the solution andthe deposit. Suitable materials for the container and for the cover maybe selected from such materials as glass, glazed ceramics, plastics,such as epoxy, polyethylene, polypropylene, polyvinyl chloride, teflon,fiberglass and other materials possessing the above qualities. The sizeof the container is at least such that the test panel is only partiallyimmersed in a predetermined volume of an electroplating solution, e.g.about up to 80-90 percent of the panel height may be submerged in thesolution. The panel is partially submerged so that a metal interface canbe exposed to the solution and to the atmosphere above it for thepurpose of a qualitative testing of the process. Typically, judgementsare made about the corrosive action of the solution on the basis of theeffects of the vapors evolved from the bath. Also some contaminants canbe identified only by their effect on the panel at the liquid/airinterface only.

Dimensions of test panel 212 (FIG. 21) being used in the HMH celltypically approximate dimensions of test panels in a Hull cell. Forexample, in a 267 ml. Hull cell, the cathode size is 3" high by 4 1/16"long (of which only up to 21/2" is immersed). In a HMH cell with 20 mmdiameter cylinder, the panel may be 41/2" high and 21/2-23/4" wide, ofwhich only 4" may be immersed. Other sizes may be used as well.

In order to compare the primary current distribution resulting from thedesign of the HMH cell with that of the standard Hull cell of FIG. 31, anumber of variants of the HMH cell shown in FIG. 1 were analized using asoftware program commercially available from L-Chem Inc., 13909Larchmere Blvd., Shaker Heights, Ohio 44120, U.S.A. Also see, W. MichaelLynes and Uziel Landau, "A Novel Adaptation of the Finite DifferenceMethod for Accurate Description of Non-Orthogonal Boundaries", TheElectrochemical Society, Fall Meeting, Chicago, 1988, Abstract 332. Theresults of the analyses are presented for each specific variant in twographs, one representing electric field distribution and the other thecurrent density distribution. The resulting current density distributionfor each variant was compared with one (FIG. 32) for the standard Hullcell. The low current density end in the HMH cell is that end of thetest panel which is the farthest from the anode.

Comparison of the current density distribution presented in FIG. 4 forthe variant with partially submerged panel, shown in FIG. 2, to thecurrent density distribution (FIG. 32) in the standard Hull cell showedthat the current density distribution of this embodiment effectivelyapproximated the current density distribution results of the Hull cell.This embodiment was considered viable for use in place of the Hull cell;however, even closer approximation was desired.

Much closer approximation was obtained by the use of a partiallysubmerged test panel with one or more horizontal baffles. These bafflesrepresent minimal restriction to the flow of liquid while acting as agood shield to the electric field. The combination with one horizontalbaffle (FIG. 5) shows that the actual current density distribution (FIG.7) is very close to the one (FIG. 32) obtainable with the standard Hullcell. FIG. 10 shows that with two horizontal baffles, 26 and 27, theactual current density distribution (FIG. 8) is even closer to the one(FIG. 32) obtainable with the standard Hull Cell. FIG. 13 shows thatwith three horizontal baffles, 26, 27 and 28, (FIG. 11) an excellentagreement with the Hull cell current density distribution (FIG. 32) wasachieved. This combination was selected for our testing and furtheroptimization.

A variant with an insulating cone-shield, 29, placed about instrument 13as presented in FIG. 14 seemed to be a natural extension of the Hullcell design, namely, because the wall of the insulating cone is at anacute angle to the low density end of the cathode. However, with acone-shield one obtains non-uniform hydrodynamics along the test panel.The cone-shield about the cylinder leads to a continuously varyinggeometry, thus, if a cone-shield is applied about the rotating cylinder,one can simultaneously obtain a whole range of hydrodynamic conditionsover the test panel. The probable type of liquid flow is schematicallypresented in FIG. 14. Since the need for the uniform and controlledhydrodynamics in the HMH cell is the dominant factor, the cone shieldwith non-uniform hydrodynamics is not a suitable arrangement. Inaddition, the use of the cone shield demonstrated (FIG. 16) an inferiorperformance for developing the current density distribution pattern forHull cell simulation.

An attempt to utilize the HMH cell with a test panel completelysubmerged in the electrolyte (FIG. 17) showed that the currentdistribution (FIG. 19) for submerged panel is clearly inferior to theone (FIG. 4) obtained with the partially submerged panel. The reasonresides mostly in the "edge effect" which takes the primary current toinfinity for a submerged electrode edge with a parallel or no insulatorat all. In a partially submerged panel this edge effect is reduced bythe solution surface which acts as an insulator perpendicular to thepanel giving current density a finite value.

The embodiment with partially immersed panel and three horizontalbaffles was rated the highest of these variants relative to the standardHull cell and was chosen for further testing. This embodiment is shownin FIGS. 20 and 21. In these FIGS. the same numerals denote the sameelements as those disclosed in FIGS. 1, 5, 8 and 11.

Cell 10 includes insulating container 11 holding electrolyte 12 in whichis suspended measuring instrument 13. The measuring element compriseselongated insulating cylinder 15, a drive shaft, 24, arranged within andcoaxially with the longitudinal central axis of the cylinder, and anodeelectrode 17 secured to shaft 24 at the bottom of the cylinder coaxiallythereof. In this embodiment, anode 17 is in the form of a thin diskwhich extends a distance of from 2 to 5 mm beyond the circumference ofthe cylinder, diameter of which may vary over a wide range, includingfrom 10 to 50 mm, preferably 20 mm, depending on the size of the celland the volume of electrolyte therein. Other forms of anode 17 areuseful as well, such as a disc or a ring having the same or smallerradial dimension as the cylinder. The important criteria is that boththe anode and the cathode are arranged coaxially on the longitudinalaxis of the cylinder in spaced relation each to another. Cathode 16includes a sleeve, 211, of refractory metal embedded into cylinder 15and is used for providing electrical contact to detachable test panel212. The test panel is of a metal which does not react with thesolution, such as copper or stainless steel. The sleeve is of refractorymetal selected from such metals as titanium, tantalum, nobium, aluminumwhich form a thin layer of oxide on their surface and thus, restrictplating of deposits on their surface. Other materials such as graphiteor glassy carbon, stainless steel, or thin coating of the refractorymetals on another metal substrate, are also useful. A slip ring, 18,mounted on the cylinder above the sleeve is electrically connected toand is used as a current carrier for the sleeve. Slip ting 18 isconnectable to a negative (cathode) terminal of an energy source, 25,e.g. a rectifier.

Drive shaft 24 is preferably of a metal and provides the positivevoltage to anode 17. Drive shaft 24 is connected to the positive (anode)side of energy source 25. Typically, the drive shaft is of a metalselected from stainless steel or brass. To avoid electrolyte attack on alower portion, 214, of the drive shaft adjacent to the anode, the lowerportion of the drive shaft may be made of the same metal as sleeve 211.Alternatively, the drive shaft may be of a rigid insulating materialwhich is non-contaminating to the solution, with electrical connectionprovided to anode 17 via a conductor (not shown) inside of the shaft orof the cylinder. The connection may be to slip ring 19 (FIG. 1) on thecylinder 15 or on that portion of the insulating shaft which projectsfrom the cylinder or via some other suitable means. Suitable materialfor the non-conducting shaft may be selected from such insulatingmaterials as epoxy, polyethylene, polypropylene, polyvinyl chloride,teflon, fiberglass and other plastic materials.

In the preferred embodiment with partially immersed test panel (cathode)and three baffles, shown schematically in FIG. 20, cell 10 includes abaffle assembly including baffles 26, 27 and 28 arranged withincontainer 11 horizontally about cylinder 13 at vertically spacedintervals each from another. For example, first baffle 26 is about 5 to15 mm from the surface of electrolyte 12, second baffle 27 is 10 to 20mm from the first baffle, and the third, lower baffle 28 is 10 to 30 mmfrom the second baffle. The baffles are made in the form of annulardiscs each having a central opening, 30, 31 and 32, respectively, sothat each successive baffle is spaced from the wall of cylinder 11, adistance greater than each preceding baffle. Central opening, 30, 31 or32, of each succeeding baffle is progressively larger so that the bodyof each successive baffle is spaced progressively greater distance fromthe cylinder. For example, first baffle 26 is spaced 5 to 15, preferably10 mm from cylinder 13, second baffle 27 is spaced 10 to 20, preferably15 mm from the cylinder and third baffle is spaced 15 to 25, preferably20 mm from the cylinder. A circular bottom anker 33 and at least twospacer connectors 34 complete the baffle assembly. Alternatively, thebaffles may be suspended from the rim of the container. Stillalternatively the baffle assembly may be in a form of a single spiralbaffle beginning at one upper corner corresponding to the position ofbaffle 26 and spirally proceeding downwardly about the instrument 13while an inwardly facing edge of the spiral baffle follows aprogressively increasing spacing distance from instrument 13. Thebaffles are made of insulating material which is unaffected by theplating reaction and which does not contaminate the plating solution.Suitable material may be selected from such insulating materials asglass, glazed ceramic, plastics, such as polyethylene, polypropylene,polyvinylchloride, teflon, fiberglass and other plastic materials.

In a HMH cell holding 500 ml of plating solution, baffles 26, 27 and 28are positioned, in a descending order, at 10, 20 and 30 mm from the topof the solution level in the container and are spaced radially 5, 10 and15 mm from cylinder 11, respectively. Outer edge of each baffle isspaced from the walls of container 11 on the order from 1 to 10 mmsufficiently to permit at least some movement of the liquid between eachbaffle and the wall of container 11. For cells with a different volume,these dimensions could vary.

In FIGS. 22, 23 and 24 are shown measurements of the thicknessdistribution across panel 22 at three separate current density levelpositions corresponding to 4, 20 and 80 ASF, respectively, at a totalapplied current of 2A. In contrast to thickness distributionmeasurements on the Hull cell panel for the same current density levelpositions (FIG. 32), these measurements indicate a very uniformdistribution of thickness across these specific current density levelson the panel. The measurements of the current density distribution basedon measurements of thickness distribution across a palladium panelplated from a proprietary plating process closely correspond to thecalculated values of current density distribution shown in FIG. 13. Thesimilarity to the actual Hull cell current density distribution (FIG.32) is quite evident.

In FIGS. 25, 26 and 27 are shown deposition patterns obtained with thestandard Hull cell (FIG. 25) and the HMH cell (FIGS. 26 and 27) from ahigh speed palladium plating bath containing 25 grams per liter Pd, pHof 7.5, with plating conducted at 40° C. The panel plated at 10 cm/secliquid velocity (FIG. 26) shows almost identical pattern as the oneobtained with the standard Hull cell. The 100 cm/sec panel (FIG. 27)shows a decreased level of burning and dullness at the high currentdensity end. This is to be expected if such features as burning anddullness on the panel are a result of the mass transport at theinterface. Lack of dullness across the panel also means that theparticular bath had no other contaminants which could be detrimental atincreased velocity.

In FIGS. 28, 29 and 30 are shown deposition patterns obtained with thestandard Hull cell (FIG. 28) and the HMH cell (FIGS. 29 and 30) from anickel electroplating bath that had copper contamination. Nickel wasdeposited at 45° C. from a plating bath containing nickel sulfamate, pHof 4.0 and 100 ppm of copper contamination. Approximately 100 ppm Cu isbarely noticeable on the panel in standard Hull cell (FIG. 28) and onthe 10 cm/sec HMH cell panel (FIG. 29). At 100 cm/sec 3/4" (1.9 cm) oflow current density side of the HMH cell panel plate had dull copperyfinish (FIG. 30). Hence, the contamination was clearly evident and therange of current densities that could be applied for its removal wouldbe easily determined.

These results indicate that HMH can give a current distributionanalogous to the one obtained with standard Hull cell but with animproved control over the transport of matter across the interface. Inaddition HMH, as a tool, opens up new avenues for future exploration andquality control of electroplating solutions.

We claim:
 1. An electroplating test cell for determining quality ofelectroplated deposits simultaneously in a wide range of currentdensities at a desired total applied current value, comprising:acontainer of non-conducting, non-contaminating material for holding anelectrolyte, and a measuring instrument which comprises: an elongatedcylinder of an electrically non-conducting material suspended within thecontainer, the central axis of the cylinder being substantially parallelto the central axis of the container, said cylinder being adapted to berotated about its longitudinal central axis, a short metallic anodeelectrode secured to a lower portion of the cylinder coaxially of thecylinder, an elongated metallic cathode electrode having a cylindricalshape secured to the periphery and extending along a major portion ofthe cylinder coaxially thereof, a lower edge of the cathode electrodebeing spaced along the said central axis from an upper edge of the anodeelectrode by a short electrically non-conducting spacer section, saidspacer section having essentially the same diameter as said cathodeelectrode, the length of said cathode electrode is such that, whenimmersed in the electrolyte, from 10 to 20 percent of the cathodeelectrode is exposed above the level of the electrolyte, and means forproviding current to the anode electrode and the cathode electrode,respectively, and in which said cathode electrode comprises a metalsleeve embedded peripherally in and coaxially with the cylinder, saidsleeve being a refractory metal selected from the group consisting ofniobium, tantalum, titanium and aluminum, and a removable test metalpanel is securable on the cylinder in electrical contact with the metalsleeve.
 2. The cell of claim 1, in which said cathode electrodecomprises titanium.
 3. The cell of claim 1, in which means is providedfor suspending the measuring instrument within said container androtating the measuring instrument about its central axis.
 4. The cell ofclaim 1, in which said anode electrode is a metal disc secured to alower portion of the spacer section transversely to the longitudinalaxis of the measuring instrument, an outer diameter of the metal discbeing greater than the outer diameter of the spacer section.
 5. The cellof claim 1, in which said anode electrode is a metal disc secured to alower portion of the spacer section transversely to the longitudinalaxis of the measuring instrument, an outer diameter of the metal discbeing essentially the same as the outer diameter of the spacer section.6. A measuring instrument for use in an electroplating test cell fordetermining quality of an electroplated deposit from an electrolytesimultaneously in a wide range of current densities at a desired totalapplied current value,the measuring instrument comprising: an elongatedcylinder of an electrically non-conducting material, said cylinder beingadapted to be rotated about its longitudinal central axis, an elongatedmetallic cathode electrode having a cylindrical shape, secured to theperiphery of the cylinder coaxially thereof and extending coaxially fora major portion of the length of the cylinder, a short metallic anodeelectrode secured to a lower portion of the cylinder coaxially thereof,the anode electrode being spaced from a lower edge of the cathodeelectrode by a short spacer section of the cylinder, an outer diameterof said spacer section being essentially the same as the outer diameterof the cathode electrode, and means for providing current to tile anodeelectrode and the cathode electrode, respectively, and in which saidcathode electrode comprises a metal sleeve embedded peripherally in andcoaxially with the cylinder, said sleeve being of a refractory metalselected from the group consisting of niobium, tantalum, titanium, andaluminum, and a removable test metal panel is securable on the cylinderin electrical contact with the metal sleeve.
 7. The instrument of claim6 in which said metal sleeve comprises titanium.
 8. The cell of claim 6,in which said anode electrode is a metal disc secured to a lower portionof the spacer section transversely to the longitudinal axis of themeasuring instrument, an outer diameter of the metal disc being greaterthan the outer diameter of the spacer section.
 9. The cell of claim 6,in which said anode electrode is a metal disc secured to a lower portionof the spacer section transversely to the longitudinal axis of themeasuring instrument, an outer diameter of the metal disc beingessentially the same as the outer diameter of the spacer section.